1. Field of the Invention
The present invention relates to the structure of the vacuum envelope for radiation image intensifying tubes such as radiological image intensifying tubes or similar electron tubes. The present invention also relates to a process for manufacturing these envelopes.
In a way known per se, the vacuum envelopes for image intensifying tubes are formed by a central body of revolution, by an input window for the passage of the radiation to be amplified, said window being connected to one end of the central body and by a transparent output window connected to the other end of the central body.
2. Description of the Prior Art
Until recently, the input windows were generally made from glass, which raised few sealing problems with the central body even when this latter was partly made from metal, for glass-metal sealing is well known to those skilled in the art. However, the use of glass for the input windows raises a number of problems. Thus, absorption of the radiation, in particular of the X rays, in the glass window varies from 15% to 25% depending on the thickness of the glass used. Now the thickness of the glass increases with the size of the tube and may vary between 2 and 3.5 mm. In addition, considerable diffusion of the radiation has been noted due to the thickness of the glass.
To overcome these disadvantages, it has been proposed to form the input window from a metal permeable to the radiation to be transmitted.
Thus vacuum envelopes have been proposed comprising a concave input window made from titanium or steel. Although this type of window may remain sufficiently thin, so absorbing or diffusing little of the radiation to be transmitted, and nevertheless sufficiently strong mechanically to withstand pressure differences, it is however necessary, because of the concave shape of the window, to extend the tube so as to be able to incorporate therein the input screen which is convex for the requirements of the electronic optical system.
Thus, it has been proposed to use windows made from aluminium or an aluminium alloy having a convex shape. In this case, different techniques are used for sealing the window to the central body.
Thus, as described in French Pat. No. 2. 482 366, sealing between the window and the central body may be achieved by thermo-compression welding. In this case, the aluminium or aluminium alloy convex window comprises an annular peripheral flange and assembly between the window and the body requires either the body to comprise an annular flange perpendicular to the axis of the tube or the use of an L or S shaped connecting ring. In fact, so as to be able to carry out thermocompression welding, the parts to be welded must be situated in a plane perpendicular to the axis of the tube so as to be able to apply a pressure between the two metals or alloys from which the window and the central body are formed.
This technique has the drawback of increasing the overall diameter of the tube. Furthermore, the thermo-compression welding process, in particular when it is used for welding aluminium to an iron alloy such as stainless steel, requires a rise in temperature and a period of contact under pressure which are time consuming. The result is that this process is industrially expensive.
Another solution of the prior art consists in forming the window by using a convex shaped piece made from a material comprising a layer of copper plated on an aluminium layer in which the copper layer is removed in the part subjected to the radiation and the aluminium layer is removed at the level of the edge formed by a flat portion surrounding the convex skull cap, thus conserving a localized covering of the two layers. The copper edge is then welded by electric arc welding along a lip formed on the central metal body which may be made from stainless steel. With this process, we find again the same problems of overall diameter of the tube as with thermocompression welding. Furthermore, it is difficult to obtain an industrially produced two layer material which always has the same reciprocal quality of adherence with vacuum tightness. In addition, metal must be removed before being able to carry out the welding.